1. Technical Field
The present invention generally relates to methods for preparing crosslinked fiber membranes and their use in separating components of a gaseous mixture.
2. Description of the Related Art
Polymeric membranes for separating mixtures of gases, such as methane and carbon dioxide are known. For example, U.S. Pat. Nos. 7,247,191; 6,932,859; and 6,755,900, disclose crosslinkable polymers and crosslinked hollow fiber membranes made from such crosslinkable polymers. These patents further disclose a crosslinkable polyimide polymer. The crosslinkable polyimide polymer can be made by monoesterifying a polyimide polymer with a crosslinking agent.
A crosslinked hollow fiber membrane can be made by forming fibers from the crosslinkable polyimide polymer and transesterifying the crosslinkable polyimide polymer within the fibers. More specifically, the crosslinkable polyimide polymer can be formed into crosslinkable fibers, which are then subjected to transesterification conditions in order to create covalent ester crosslinks within the fibers. Such fibers can be hollow fibers or other types of fibers. Crosslinked hollow fiber membranes can be incorporated into a separation module. Other types of membranes for separation include flat sheet separation membranes or flat stack permeators.
Integrally skinned hollow fiber membranes can be formed by contacting the polymer solution with a non-solvent and forming the membrane in a one step process. On contact with the non-solvent, mass transfer takes place between the non-solvent from the coagulation bath and the solvent in the nascent membrane resulting in micro-phase separation within the membrane. Depending on the pathway of phase separation, a dense layer, also called the skin layer, is believed to form on the surface of the membrane. The skin formation is hypothesized to occur when solvent outflow from the membrane exceeds the non-solvent inflow resulting in delayed demixing. This process increases the concentration of the polymer at the membrane-coagulant interface and forms the skin. An evaporative step in the air gap can be included prior to the phase separation step to enhance skin formation by the evaporation of the volatile solvent from the nascent membrane followed by a rapid phase separation of the underlying region to form a highly porous support.
Polymer solutions used in hollow fiber membrane spinning consist of polymer, solvents, non-solvent and additives. When the number of components exceeds three, a pseudo-ternary phase diagram of more than three components can be devised by dividing the components into categories of polymer, solvent and non-solvent. Within each category, the components can be fixed in ratio to each other to restrict solvency and/or non-solvency power. This approach based on fixed ratios enables holding solvency parameters constant for the solvents and nonsolvents that can be explored in the system and a binodal (set of concentrations separating the single phase and two phase regions) obtained.
While not wishing to be bound by any particular theory, ternary phase diagrams can be developed (1) by the titration of the polymer solution with non-solvent, (2) through the use of the three-phase Flory-Huggins theory for polymer solutions, and (3) by inspection of polymer solutions of various compositions of polymer/solvent/nonsolvent. Depending on the polymer viscosity in solution, the dope compositions are made to cover the region of interest for fiber spinning (usually 20 to 40 wt. % polymer). The binodal curve can be generated by making small samples (10 to 15 gram) of various compositions and visually inspecting them for phase separation.
Once the binodal has been identified, three factors taken into consideration when determining the dope formulation are: (1) proximity of the dope composition to the binodal, (2) osmotic pressure of the solution, and (3) polymer solution viscosity.
The proximity of the polymer solution composition to the binodal and osmotic pressure of the solution determine the kinetics of membrane formation and membrane morphology. Osmotic pressure has earlier been suggested as the cause for the large finger/tear shaped voids (macrovoids) found in certain membranes. To describe the phase separation of the polymer solution (in forming the membrane), a ternary diagram can be formed which groups all the solvents, nonsolvents and additives into the solvent category, and depicts the coagulant (typically water) in the nonsolvent category. Based on the proximity of the polymer solution to the binodal, the quantity of coagulant required to phase separate the polymer solution can be determined. Since the penetration of the coagulant into the polymer solution is limited by the rate of diffusion, the distance of the polymer solution from the binodal and the osmotic pressure driving force determines the rate and type of phase separation. Compositional change on the ternary phase diagram (FIG. 1) from point 1 (original polymer solution) to point 2 is hypothesized for the skin and from point 1 to an arbitrary position 3 (in the spinodal region) for the support layer of the membrane. The objective is to drive phase separation of the support layer through spinodal decomposition mechanism to form a highly porous support with little or no resistance to gas flow.
The minimum polymer solution viscosity depends on the strength of the polymer solution strand which undergoes elongation (under gravity) that takes place after the fiber exits the spinneret. Based on the air gap and draw ratio, this minimum viscosity must be defined for each polymer/solvent/nonsolvent system. A higher viscosity can be achieved by increasing the polymer concentration in the polymer solution or by adding viscosity enhancers, like lithium nitrate (LiNO3) and carboxylic acids which complex with the common spinning solvents (i.e. N-methyl-2-pyrrolidone). Although a high polymer concentration is generally required to promote skin growth and increase viscosity for spinning, it is believed that too high of a polymer concentration would reduce porosity in the support layer and form a support layer with substantial resistance to gas flow which is undesirable.
Solvents and non-solvents are selected, in part, for their miscibility with the aqueous coagulant. Another factor for consideration in the selection of the polymer solution solvent is the generation of osmotic pressure during phase separation. The osmotic pressure is a function of the thermodynamic activities of the solvent and coagulant non-solvent, and is believed to be a factor in the formation of macrovoids.
The crosslinked hollow fiber membranes have good permeability and selectivity. The crosslinked hollow fiber membranes also have good resistance to plasticization. Plasticization occurs when one or more components of a fluid mixture causes the polymer to swell thereby altering the properties of the membrane. For example, polyimides are particularly susceptible to plasticization by carbon dioxide. Subjecting the fibers to transesterification conditions to crosslink the crosslinkable polyimide polymer within the fibers increases both resistance to plasticization and selectivity.
The above referenced patents disclose the use of sufficiently high molecular weight polyimide polymers to accommodate for molecular weight loss during the monoesterification process. However, it is difficult to produce crosslinkable polyimide polymers having such a high molecular weight. Therefore, there is a need for a method of making a crosslinkable (i.e., monoesterified) polyimide polymer that reduces or eliminates the loss of molecular weight during the monoesterification process, i.e., a high molecular weight, monoesterified polyimide polymer, while having improved strength, flexibility, and/or spinnability.